As an example of a known Faraday cage, Patent Document 1 describes a Faraday cage including an insulated container, and a suction nozzle (conductive container) disposed inside the insulation container, which has an intake vent part and an exhaust vent part sandwiching therebetween a filter for collecting toner, whereby the electric charge of the toner inside the suction nozzle serving as a conductive container is measured. In such a Faraday cage is measured the electric charge of the toner sucked into the suction nozzle in the insulation container. The lid of the insulation container is removed to take out the suction nozzle from the insulation container, and the weight of the suction nozzle containing the toner is measured. The per-unit weight electrostatic charge of the toner is then calculated by dividing the measured electric charge by the difference between the weight of the suction nozzle alone, which is measured beforehand, and the measured weight of the suction nozzle containing the toner.